Different methods for detecting the presence of foreign matter on a glass surface are known. In most cases, advantage is taken of the fact that the transmission of a wave (optical, elastic or other wave) in the material of the glass can be found to be noticeably affected by a modification of the medium in which the glass is located. Thus, in the case of a windshield for an automotive vehicle, the transmission of a luminous wave by multiple reflection in the thickness of the glass may be more or less attenuated as a function of the surface state of one of the boundary surfaces of the windshield.
French patent 2.648.096 proposes an automatic cleaning system using optical detection means applying this principle of attenuation.
This latter principle still remains valid in the case of an elastic wave and ultrasonic detection systems have often been proposed.
German patent DE 40 33 975, for example, detects the variations in amplitude of an ultrasonic signal received at a detector arranged at a certain distance from the transmitter.
An analogous principle of detection by amplitude variation is employed in the French patent 91 05784 in which the functions of transmission and reception are effected by one and the same transducer.
In Japanese patent application JP 59-192 651, it is the existence of supplemental pulses in the ultrasonic signal received by the detector which enables deduction of the presence of foreign bodies. Such additional pulses arise from the fact that the presence of foreign bodies generates supplemental wave transmission paths.
The known arrangements cited hereinabove employing an ultrasonic detection system nevertheless exhibit the drawback of operating at a relatively high frequency, typically on the order of 1 to 100 MHz; and whether such frequency be emitted as a continuous wave or in the form of pulses, the processing circuits must also be able to work at high frequency. This condition forbids envisaging for such applications the use of inexpensive circuits such as those formed with the help of slower technologies, for example MOS or CMOS technologies. Now it is readily conceived that a high cost of obtaining such circuits constitutes a brake on the generalization of use of such automatically controlled cleaning arrangements which represent however, for the user, an undeniable advantage as much in view of comfort as that of security.
Thus a purpose of the invention is to obtain an automatically controlled cleaning arrangement intended for the elimination of foreign matter from the surface of a glass using an ultrasonic detector and not showing the drawbacks mentioned hereinabove.
Another purpose of the invention is to obtain a cleaning arrangement the detector of which functions at a relatively low frequency.
Still another purpose of the invention is to obtain a cleaning arrangement the cost of which is lower than that of systems of the prior art.